As communication networks have continued to expand and diversify to meet the needs of a variety of subscriber/users, the continued successful operation of relay satellites, which constitute a critical component of each network link, has continued to draw increasing attention, especially with respect to the matter of power allocation among the links. In order for any relay link to operate successfully over a variety of link conditions, i.e. to accommodate changing levels of signal attenuation caused, for example, by rainfall between the relay satellite and the earth station, the satellite TWT power allocated to each downlink terminal is tailored to provide a prescribed degree of excess power (rain fade margin). Typically, this power differential may be on the order of 6 dB higher than that required to close the link in clear weather. Because heavy rainfall is infrequent and all terminals do not experience rainfall simultaneously, this rain fade margin is wasted most of the time.
In an effort to circumvent this highly inefficient allocation of effectively unused excess power, adaptive link power control (ALPC) schemes such as described in U.S. Pat. Nos. 4,261,054 to Scharla-Nielsen and 4,228,538 to Scharla-Nielsen et al and assigned to the Assignee of the present application, have been proposed. In an ALPC satellite network, each link monitors its received signal quality and sends appropriate power control commands to the transmitter terminal in an attempt to maintain desired link performance during rain fades. In response to these power control commands the uplink transmitter causes power to be drawn from a common or shared power pool that is normally held in reserve in the satellite TWT until it is actually required by individual downlinks. Because of the statistical nature of the occurrence and intensity of rain fades, the size of this power pool o rain margin in the TWT is considerably less than the above-referenced 6 dB figure, so that the satellite network can support more terminals and/or higher data throughput. (For a description of additional examples of schemes for circumventing the problem of rain fades attention may be directed to the U.S. Pat. No. 3,667,043, to Ahlgren, Arens et al U.S. Pat. No. 4,004,224, Welti U.S. Pat. No. 3,917,998, Thomas U.S. Pat. No. 4,038,600, Acampora U.S. Pat. No. 4,309,764, Acampora et al 4,301,533 and Mori 3,676,778.)
Now although the ALPC approach offers a reduction in wasted power and thereby an improvement in throughput capacity, it subjects the network to a potential "lockup" condition, in which the satellite TWT intermodulation product power received by a terminal can become comparable to the terminal's internally generated thermal noise. In this event, successive ALPC cycles drive the TWT into saturation, after which all network links exhibit unsatisfactory performance due to excessive intermodulation product power generated in the satellite TWT.